The present invention relates to a semiconductor protection device, and more specifically relates to a particular type of the semiconductor protection device for use in protection of a wire communication instrument etc. from an abnormal voltage etc. which is applied through a communication cable.
FIG. 1A is a plan view showing such type of the conventional semiconductor protection device, FIG. 1B is a sectional view taken along the line X--X' of FIG. 1A, and FIG. 1C is another sectional view taken along the line Y--Y' of FIG. 1A. As shown in these figures, the conventional semiconductor protection device is comprised of an n.sup.- type semiconductor region 1 having a necessary effective thickness, a p.sup.+ type semiconductor region 2 disposed to form a pn junction in contact with an entire face of one of the two major faces of the n.sup.- type semiconductor region 1, a p type semiconductor region 3 disposed to selectively form a pn junction in the other of the two major faces of the n.sup.- type semiconductor region 1, which is opposite to said one major face on which the first-mentioned pn junction is formed with the p.sup.+ type semiconductor region 2, an n.sup.+ type semiconductor region 4 disposed within the p type semiconductor region 3 to form selectively a pn junction, an n type impurity diffusion region 5 formed in the device surface along an entire length of the pn junction between the n.sup.- type semiconductor region 1 and the p type semiconductor region 3, by implanting therealong an n type impurity at a suitable density, a first electrode 6 in ohmic contact with the p.sup.+ type semiconductor region 2, a second electrode 7 provided in ohmic contact with both of the p type semiconductor region 3 and the n.sup.+ type semiconductor region 4, and an insulating film 8 provided to protect the device surface.
When an abnormally high voltage is applied between the pair of first and second electrodes 6 and 7 of the device such that the first electrode 6 is held at a positive potential, avalanche phenomenon is induced in the impurity diffusion region 5 as the applied voltage exceeds a certain level because the pn junction formed between the n.sup.- type semiconductor region 1 and the p type semiconductor region 3 has the lowest breakdown voltage at a portion into which the n type impurity of suitable density is implanted to form the n type impurity diffusion region 5. Consequently, an electric current starts to flow from the first electrode 6 to the second electrode 7 through the p.sup.+ type semiconductor region 2, n.sup.- type semiconductor region 1, and p type semiconductor region 3 so that the semiconductor protection device turns to ON-state in response to this avalanche current.
FIG. 2 shows general voltage-current characteristics observed in this state. In the FIG. 2 graph, a breakdown voltage V.sub.BR denotes a voltage at which the pn junction is broken down to start flowing of an electric current, a breakover current I.sub.BO denotes a maximum electric current flowing immediately before the protection device switches from the OFF-state to the ON state, and a holding current I.sub.H denotes a minimum electric current needed to hold the protection device in the ON-state.
FIG. 3 is an equivalent circuit diagram of the conventional semiconductor protection device. In the diagram, numeral 9 denotes a pnp transistor comprised of the p.sup.+ type semiconductor region 2, the n.sup.- type semiconductor region 1 and the p type semiconductor region 3. Numeral 10 denotes an npn transistor comprised of the n.sup.- type semiconductor region 1, the p type semiconductor region 3 and the n.sup.+ type semiconductor region 4. Numeral 11 denotes a zener diode formed of the pn junction comprised of the semiconductor regions 1-3 within the n type impurity diffusion region 5. Numeral 12 denotes an anode terminal composed of the first electrode 6. Numeral 13 denotes a cathode terminal composed of the second electrode 7. Character R denotes a parasitic resistance in the semiconductor region 3.
The holding current I.sub.H is one of the important characteristic values in the semiconductor protection device. Generally, this value should be set in a higher range for the better performance. For example, in case that a lightning surge is applied through a communication line into a semiconductor protection device used in a telephone switchboard during the course of operation, the semiconductor protection device turns to the ON-state to pass an abnormal current to the ground. Concurrently, the operating signal current also flows through the semiconductor protection device to the ground. Accordingly, the operating signal current tends to continuously flow through the semiconductor protection device even after the abnormal current due to lightning surge has completely passed away to the ground. In such case, if the holding current I.sub.H of the semiconductor protection device is lower than the operating signal current, the semiconductor protection device cannot return to the OFF-state due to the operating signal current.
On the other hand, the breakover current I.sub.BO is another of the important characteristic values. Since this value represents a degree of switchability of the semiconductor protection device to the ON-state, this value should be set in a smaller range for the better performance. Consequently, the breakover current I.sub.BO should be set as small as possible, while the holding current I.sub.H should be set as great as possible in the semiconductor protection device.
For this, as illustrated in an equivalent circuit diagram shown in FIG. 4, it might be advantageous to form a resistor switchable between R.sub.off and R.sub.on for branching a base current of the npn transistor 10. Namely, the resistor R.sub.on is effective when the transistor 10 turns to the ON-state and has a relatively great resistance so as to facilitate supply of an electric current to the base of the npn transistor 10. On the other hand, the resistor R.sub.off is effective when the transistor 10 returns to the OFF-state and has a relatively small resistance so as to suppress supply of an electric current to the base of the npn transistor 10 to facilitate turning-off of the transistor.
However, according to the inventors' experiments, it has been found in construction of the device of FIG. 1 that either component of the breakover current I.sub.BO due to breakdown of the semiconductor protection device and the holding current I.sub.H flowing at the last end under the ON-state during gradual reduction of a voltage passes through the pn junction formed between the n.sup.- type semiconductor region 1 and the p type semiconductor region 3 at the same curved portion observed on the plan view of FIG. 1. Namely, as indicated by the arrow C in FIG. 1, the electric current flows across the pnp triple layers from the first electrode 6 to the second electrode 7 through sequentially the p.sup.+ type semiconductor region 2, the n.sup.- type semiconductor region, 1, then the curved portion, when observed in the plan view of FIG. 1, of the pn junction formed between the n.sup.- type semiconductor region 1 and the p type semiconductor region 3, and lastly the p type semiconductor region 3 in a horizontal direction under the n.sup.+ type semiconductor region 4.
For this reason, in the construction of the device of FIG. 1, either of the equivalent resistors R.sub.on and R.sub.off indicated in FIG. 4 has the same resistance value defined from the curved portion, when observed in the plan view, of the pn junction formed between the n.sup.- type semiconductor region 1 and the p type semiconductor region 3 to an ohmic contact region between the p type semiconductor region 3 and the second electrode 7 through a transverse length of the p type semiconductor region 3. Stated otherwise, the resistors R.sub.on and R.sub.off actually cannot be formed separately from each other in the conventional construction. As shown in FIG. 3, its equivalent circuit contains a single of the base resistor R. Such type of the semiconductor protection device having the FIG. 3 equivalent circuit cannot suppress the breakover current I.sub.BO in a relatively low level while maintaining the holding current I.sub.H in a relatively high level.